SEWAGE TREATMENT — KEEFER 385 



DISPOSAL IN WATER 



Where sewage-treatment plants are situated near sufficiently large 

 bodies of water that can dilute and carry away the sludge without 

 creating unsanitary conditions, this method of disposal has been 

 adopted with considerable success. The sludge is pumped into spe- 

 cially designed steamers that transport and dump it at some remote 

 spot. Sludge has been disposed of in this way for many years at 

 Elizabeth, N. J., and New York, N. Y., in this country, and in London, 

 Manchester, and Glasgow in Great Britain. 



DISPOSAL ON LAND 



The most generally adopted method of sludge disposal is to apply it 

 to the land, either wet or dry, raw or digested. The methods of land 

 application include lagooning, trenching, flowing on land, depositing 

 in dumps, and distributing for fertilizing purposes. 



Lagoons consist of natural or artificial depressions in the ground 

 enclosed by earth dykes. Wet sludge in a semidigested or digested 

 condition is pumped into the lagoons. The disadvantages associated 

 with lagoons is that they occupy large areas of ground, they are fre- 

 quently odorous, and they are usually a temporary expedient. 



Another method of disposal is to pump the wet sludge into trenches, 

 which are then covered with earth. After the sludge has decomposed 

 and the water has drained away, it is often possible to reuse the same 

 land. 



At a few plants wet digested sludge is pumped through pipes and 

 channels and allowed to flow over the ground. The sludge serves 

 to irrigate and fertilize the soil. Any crops that are grown should 

 be such that they will not be contaminated by the sludge. 



At many sewage plants such as those serving Baltimore, Md., and 

 Washington, D. C, sludge that has been dewatered on sludge-drying 

 beds or by vacuum filters is deposited in dumps. Sludge dumps are 

 unsightly and frequently produce odors ; therefore, they are not looked 

 upon with favor by sanitary engineers. 



Sludge that has been dewatered on sludge-drying beds or by vacuum 

 filters is widely used by gardners and farmers as a soil conditioner. 

 The water content of the material, which may vary from about 50 to 

 75 percent, is such that it can be readily applied to and incorporated 

 in the soil. The chief advantages of sludge are that it has excellent 

 moisture-retaining characteristics and contains a considerable per- 

 centage of humus. The nitrogen and phosphates in sludge are low. 

 Primary tank sludge contains about 1 to 2 percent of nitrogen on the 

 dry basis, and activated and humus-tank sludge contains 4 to 6 per- 

 cent. The phosphates in sludge vary from approximately 2 to 3 per- 

 cent. At many sewage plants a small charge is made for air-dried 



